Polyolefins, namely polyolefin plastics and elastomers, are useful in any number of everyday articles. Semicrystalline polyolefins, in particular, are used in such applications because they are thermoplastic, meaning, among other things, that they exhibit some useful degree of crystallinity in the solid state. This crystalline nature allows them to form useful articles without crosslinking due to favorable combination of processing characteristics, physical properties, and aesthetics. For example, such materials can be formed into pellets for ease of handling and are processed using standard plastic-industry equipment such as extruders. Thermoplastic polyolefins are fundamentally different from polyolefin thermoset materials, such as ethylene-propylene elastomers or rubbers, including ethylene-propylene-diene monomer (EPDM) versions, which have little to no crystallinity, very high molecular weights, and are typically cross-linked to form useful articles. Such materials are generally not pelletizable and are processed using standard rubber-industry equipment such as roll mills.
However, one drawback to many thermoplastic polyolefins, especially propylene-rich ones, is their relatively high glass transition temperature. This characteristic makes these polyolefins brittle, especially at low temperatures. Many applications of thermoplastic polyolefins benefit from having useful properties over a broad range of temperatures; consequently, there is a need to provide polyolefins that can maintain desirable characteristics such as high or low temperature performance, etc., while maintaining or improving upon the impact strength and toughness at lower temperatures. In particular, it would be advantageous to provide thermoplastic polyolefins possessing improved toughness, flexibility, and or high use temperature without sacrificing their other desirable properties, including optical properties such as high clarity and low haze. Preferably, the thermoplastic polyolefin will also exhibit high crystallization temperature and high crystallization rate, to allow fast processing. Furthermore, articles fabricated from the thermoplastic polyolefin should have high clarity, low haze, and good surface aesthetics, particularly without oily or sticky substances on the surface.
Specifically, there is a need for thermoplastic polyolefin compositions, especially polypropylene and polyethylene compositions, that can be used in such applications as food containers, health care products, durable household and office goods, squeeze bottles, clear flexible film and sheet, automotive interior trim and fascia, wire, cable, pipe, and toys. Even more specifically, a thermoplastic polyolefin composition with clear, homogenous appearance after molding is needed. A plasticized polyolefin according to this invention can fulfill these needs.
Addition of a low molecular weight, amorphous substance to a polyolefin is one way to modify its properties and processing characteristics. Some patent disclosures directed to such an end are U.S. Pat. Nos. 3,201,364; 3,415,925; 4,073,782; 4,110,185; 4,132,698; 4,210,570; 4,325,850; 4,960,820; 4,774,277; 5,869,555; 6,465,109; EP 0448259, FR 2094870, and JP 09-208761. These disclosures are directed to polyolefins blended with materials such as mineral oils which often contain substantial concentrations of unsaturation, aromatic groups, naphthenic groups, and/or other functional groups. Addition of mineral oils in polyolefin elastomers, which have little to no crystallinity and very high molecular weights, is also well known; see e.g., RUBBER TECHNOLOGY HANDBOOK, Werner Hoffman (Hanser, N.Y., 1989), p. 294-305.
Addition of mineral oil tends to improve the flexibility of a polyolefin, which identifies such compounds as “plasticizers” under the commonly accepted definition; that is, a substance that improves the flexibility, workability, or distensibility of a plastic or elastomer. Mineral oils are also added to polyolefins as extender oils or processing oils, as well as for other purposes. However, use of these additive compounds often does not preserve the optical properties (e.g., color and/or transparency), or low odor, or upper (lower) use temperature ranges of the polyolefin, among other things. In addition, such additive compounds typically have high pour points (greater than −20° C., or even greater than −10° C.), resulting in little or no improvement in low temperature properties of the polyolefin. Another drawback is that all or some of the mineral oil can migrate to a surface and evaporate at an unacceptably high rate, which results in deterioration of properties over time, among other things. If the flash point is sufficiently low (e.g., less than 200° C.), the compound can cause smoking and be lost to the atmosphere during melt processing. It can also leach out of the polyolefin and impair food, clothing, and other articles that are in contact with the final article made from the polyolefin composition. It can also cause problems with tackiness or other surface properties of the final article. What is needed is a compound which imparts superior low temperature properties while also exhibiting low bloom, migration, leaching, and/or evaporation behaviors.
Yet another shortcoming of mineral oils is that they often contain a high (greater than 5 wt %) degree of functionality due to carbon unsaturation and/or heteroatoms, which tends to make them reactive, thermally unstable, and/or incompatible with polyolefins, among other things. Mineral oils may in fact contain thousands of different compounds, many of which are undesirable for use in polyolefins due to molecular weight or chemical composition. Under moderate to high temperatures these compounds can volatilize and oxidize, even with the addition of oxidation inhibitors. They can also lead to problems during melt processing and fabrication steps, including degradation of molecular weight, cross-linking, or discoloration. They may also impart an undesirable odor.
These attributes of common additive compounds like mineral oils limit the performance of the final polyolefin composition, and therefore its usefulness in many applications. As a result, they are not highly desirable for use as modifiers for thermoplastic polyolefins. What is needed is a modifier that does not suffer from these deficiencies. Preferably, what is needed is a modifier that allows the formulation of thermoplastic polyolefin compositions with improved softness, flexibility (lower flexural modulus), and impact toughness especially at low temperatures (below 0° C.), while not materially degrading the thermal resistance and with minimal migration of low molecular weight substances to the surface of fabricated articles. Ideally, the modifier would have a low pour point, while still of sufficient molecular weight to avoid unacceptable exudation and extraction. It would also not contribute to deterioration of performance attributes such as optical properties, color, smell, thermal stability, and/or oxidative stability. Preferably, the glass transition temperature of the modified polyolefin composition would be lower than that of the unmodified polyolefin. A plasticized composition according to this invention can fulfill these needs.
It would be particularly desirable to modify thermoplastic polyolefins by addition of a simple, non-reactive compound such as paraffin liquid. However, it has been taught that addition of aliphatic or paraffinic compounds impairs the properties of polyolefins, and is thus not recommended; see, e.g., CHEMICAL ADDITIVES FOR PLASTICS INDUSTRY (1987, Radian Corp., Noyes Data Corporation, NJ), p. 107-116. Other background references of interest include U.S. Pat. No. 6,639,020 and ADDITIVES FOR PLASTICS, J. Stepek, H. Daoust (Springer Verlag, New York, 1983), p. 6-69.
Certain mineral oils, distinguished by their viscosity indices and the amount of saturates and sulfur they contain, have been classified as Hydrocarbon Basestock Group I, II or III by the American Petroleum Institute (API). Group I basestocks are solvent refined mineral oils. They contain the most unsaturates and sulfur and have the lowest viscosity indices. They define the bottom tier of lubricant performance. Group I basestocks are the least expensive to produce, and they currently account for abut 75 percent of all basestocks. These comprise the bulk of the “conventional” basestocks. Groups II and III are the High Viscosity Index and Very High Viscosity Index basestocks. They are hydroprocessed mineral oils. The Group III oils contain less unsaturates and sulfur than the Group I oils and have higher viscosity indices than the Group II oils do. Additional basestocks, named Groups IV and V, are also used in the basestock industry. The five basestock groups are described, for example in SYNTHETIC LUBRICANTS AND HIGH-PERFORMANCE FUNCTIONAL FLUIDS (Leslie R. Rudnick & Ronald L. Shubkin, ed. Marcel Dekker, Inc. 1999; p. 409) as being:    Group I—mineral oils refined using solvent extraction of aromatics, solvent dewaxing, hydrofining to reduce sulfur content; produces mineral oils with sulfur levels typically greater than 0.1%, saturates levels of 60-80%, and viscosity index (VI) of about 90;    Group II—mildly hydrocracked mineral oils with conventional solvent extraction of aromatics, solvent dewaxing, and more severe hydrofining to reduce sulfur levels to less than or equal to 0.1% (typically 0.03%) as well as removing double bonds from some of the olefinic and aromatic compounds; saturate levels are greater than 95-98%, and VI is approximately 90-100;    Group III—severely hydrotreated mineral oils with saturates levels of some oils virtually 100%; sulfur contents are between 0.001 and 0.01%, and VI is in excess of 120;    Group IV—poly(alpha-olefin)s—synthetic fluids most commonly manufactured by catalytic oligomerization of linear olefins having 6 or more carbon atoms, but more generally meaning saturated olefin oligomers produced by oligomerizing C4 and greater alphaolefins; and    Group V—esters, polyethers, polyalkylene glycols, etc.—generally, all other synthetic basestocks not included in Groups I, II, III and IV.
Prior attempts of adding mineral oils to polyolefins to modify properties involve for the most part addition of Group I and Group II mineral oils. Even in cases where the mineral oil is not identified by an API Group classification, such as the case for so-called “process oils,” “technical white oils,” “food grade oils,” etc., such mineral oils are still readily categorized into two classes based on VI alone: those with VI less than 120 (similar to Group I and Group II mineral oils), and those with VI of 120 or greater. Certain aspects of the present invention ideally pertain to substances with a VI of 120 or greater, which excludes Group I and Group II mineral oils and any other mineral oils with VI less than 120.
Examples of thermoplastic polyolefins combined with paraffinic liquid plasticizers for non-adhesive applications include the following:
U.S. Pat. No. 4,536,537 discloses polypropylene compositions that comprise LLDPE having a density of 0.912 to 0.935 g/cm3 or polybutene and poly-α-olefin liquid having a kinematic viscosity of about 2 cSt to about 6 cSt at 100° F./38° C.; those with viscosity greater than about 2 cSt are reported to “not work” (col 3, ln 12).
WO 98/44041 discloses blend compositions that comprise a chlorine-free polyolefin and poly-α-olefin oligomers having a kinematic viscosity at 100° C. of about 4 cSt to about 8 cSt for a sheet-like structure, especially a floor covering.
WO 2002/18487 and WO 2003/48252 disclose polypropylene compositions that comprise 10 to 30 wt % of vulcanized or unvulcanized polyolefin elastomers, especially EPDM or styrene-ethylene-butene-styrene (SEBS) block-copolymers, and poly-α-olefin oligomers having a kinematic viscosity at 100° C. of about 4 cSt to about 8 cSt.
U.S. Pat. No. 4,645,791, JP 07292167, EP 0315363, and WO 2002/31044 all disclose poly-α-olefin type materials in EPDM compositions.
JP 56095938 discloses polypropylene compositions that comprise olefin oligomer plasticizers mixed with polyolefin granules.
WO 2004/14998 discloses propylene-based polymer compositions that comprise various plasticizers, including poly-alpha-olefins. Some compositions contain a nucleating agent.
In another area, paraffins and traditional lubricant basestocks have recently been used as polymer modifiers. WO 2004/014997, US 2004/054040 (U.S. Ser. No. 10/634,351), WO 2004/014998 (also noted above), and US 2004/106723 (U.S. Ser. No. 10/640,435) disclose blends of polyolefins such as polypropylene and or polybutene with various liquids (such as isoparaffins, n-paraffins, polyalphaolefins, highly refined Group III basestocks, polybutenes, Gas-To-Liquid type molecules, and others) used as non-functional plasticizers (NFP's). These compositions are reported to have superior properties, such as good flex and stiffness combinations as well as low exudation of the NFP. Other examples of NFP materials used as plasticizers in many applications include WO 2005/080495; US 2005/148720 (U.S. Ser. No. 11/054,247); US 2004/0260001 (U.S. Ser. No. 10/782,228); US 2004/0186214 (U.S. Ser. No. 10/782,306); U.S. Ser. No. 60/649,266 filed Feb. 2, 2005; U.S. Ser. Nos. 11/118,925, 11/119,072, and 11/119,193, all filed Apr. 29, 2005; U.S. Ser. No. 60/649,107 filed Jun. 24, 2005; GB 0511319.6 and GB 0511320.4, both filed Jun. 3, 2005; and U.S. Ser. No. 60/699,718 filed Jul. 15, 2005.
Other references of interest include: GB 1329915; JP 01282280, JP 69029554, WO 2001/18109; EP 0300689; and EP 1028145.
The above examples show that certain paraffinic liquid plasticizers modify the properties (e.g., flexibility and low-temperature impact strength) of polyolefins, in particular polypropylene. However, it has been found that, under certain conditions, plasticization causes certain types of thermoplastic polyolefins, especially polypropylene, to exhibit undesirable optical and/or tactile properties. For example, isotactic polypropylene can develop a distinct hazy region in the interior of injection-molded specimens after aging (seconds to minutes); and random copolymer polypropylene can develop an oily feel on the surface of compression-molded specimens after aging (hours to days). Under an optical microscope, a hazy region contains an inhomogeneous and irregular distribution of amorphous domains, likely rich in the liquid plasticizer, that are large enough to scatter light. Similarly, an oily feel is likely due to a plasticizer-rich layer on the surface. The conditions required for the onset of undesirable optical and/or tactile behavior depend on the nature of the molding process, the type of polyolefin, the type of plasticizer, and the concentration of plasticizer (typically, a concentration higher than some critical level). What is needed is a means to modify the plasticized composition so as to ensure satisfactory aesthetics in a molded article.
Addition of a nucleating agent to the thermoplastic polyolefin/plasticizer blend, as demonstrated in the present invention, satisfies this need. Specifically, appropriate selection and use of a nucleating agent increases the robustness of the plasticization process, which allows for increased plasticizer concentrations (and therefore greater modification of mechanical properties and processing characteristics) without the appearance of internal haze or surface oiliness, thereby retaining acceptable optical and tactile properties.